This invention relates to a speed-changing device for electric ceiling fans, and more particularly to a tapped coil speed-changing device designed for eliminating the motor noise of an electric ceiling fan during speed change.
Conventionally, speed changing devices for electric ceiling fans are usually classified into two types; capacitor and tapped.
In the capacitor type of speed-changing devices, although no noise of the associated motor is produced during speed change of an electric ceiling fan, the manufacturing cost is very high simply because each speed requires a capacitor of different value. Moreover, the phase angle between the capacitance, resistance and current of the speed changing device is unstable and causes the motor's temperature to rise, which is not satisfactory in practical usage
Concerning the known tapped type of speed changing devices, the structure and the speed changing conditions are as shown in
FIGS. 1A, 1B, 1C, 1D, 1E and 1F.
Shown in FIGS. 1 A and 1B is a high speed condition of the motor with a tapped type of speed-changing device, wherein the current directions of all the coils are indicated in arrows drawn in accordance with the "left hand rule" (i.e. when the fingers are placed around the coil, the thumb points in the direction of the magnetic north produced in the coil, and the rest of the fingers point in the direction of the current flow). As illustrated in the drawing, when the motor is running at a high speed, the distribution of the magnetic lines in the stator of the motor is uniform and symmetrical; thus, the magnetic effect between the stator and rotator is excellent, and the motor is running in a stable and noiseless condition.
However, as the motor revolution is changed from the high speed to a medium or a low speed, as shown in FIGS. 1C, 1D and 1E, 1F the distribution of the magnetic lines (also drawn on the basis of the "left hand rule") shows that part of the current flow directions are opposite to that of the high speed revolution due to the fact that central taps are made in the coils. As a result, the original uniform and symmetric magnetic lines existing in high speed revolution are suddently changed to an irregular condition, so that the revolution is not only noisy but also unstable.